Green extension circuit for traffic control

ABSTRACT

A self-adaptive traffic light control system for varying the green light period as a function of the traffic volume. The system employs a first resistor and a first capacitor serially connected across a DC voltage source and a second resistor and a second capacitor also serially connected across said source. The first series circuit has a lower RC time constant than the second series circuit. The junction points of each series circuit are interconnected by a diode. The first capacitor is discharged at the start of a green light period and is rapidly discharged each time a vehicle passes a sensing device in the road. The second capacitor is charged at a low constant rate which is not changed for high volume traffic. When the traffic is light, the first capacitor is allowed to charge up rapidly and, by means of the diode, acts upon the second capacitor to reduce its charge time and thus reduce the total green light period.

United States Patent [72] Inventor Jean Dressayre Orsay, France [21] Appl No 787,046 [22] Filed Dec. 26, 1968 [45] Patented Aug. 10, 1971 l 73] Assignee U. S. Philips Corporation New York, NY. [32] Priority Dec. 21,1967 [33] France [3 l] 133350 [54] GREEN EXTENSION CIRCUIT FOR TRAFFIC CONTROL 5 Claims, 2 Drawing Figs.

[52] [1.8. CI 340/31, 320/1 [51] lnt.Cl.... G083 1/08, H02 5/00 [50] Field of Search 340/31 A, 37; 320/ l [56 1 References Cited UNITED STATES PATENTS 3,24 l ,l09 3/l966 DuVivier 340/37 3,4l6,l30 12/1968 Brockett 3,466,599 9/1969 Clark etal.

ABSTRACT: A self-adaptive traffic light control system for varying the green light period as a function of the traffic volume. The system employs a first resistor and a first capacitor serially connected across a DC voltage source and a second resistor and a second capacitor also serially connected across said source. The first series circuit has a lower RC time constant than the second series circuit The junction points of each series circuit are interconnected by a diode. The first capacitor is discharged at the start of a green light period and is rapidly discharged each time a vehicle passes a sensing device in the road. The second capacitor is charged at a low constant rate which is not changed for high volume traffic. When the traffic is light, the first capacitor is allowed to charge up rapidly and, by means of the diode, acts upon the second capacitor to reduce its charge time and thus reduce the total green light period.

PATENTEI] mm om 3.599.144

INVENTOR.

JEAN DRESSAYRE jW A Z AGENT GREEN EXTENSION CIRCUIT FOR TRAFFIC CONTROL This invention relates in general to a traffic light control system and more particularly to a self-adaptive system that is capable of relieving traffic congestion on roads in urban areas and, in particular to the regulation of the traffic at a road crossing as a function of the traffic existing at a given instant on each of the roads traversing said crossing.

The present known adaptive systems are in general based on the principle of the prolongation of the time during which a green light permits the passage of the vehicles on the corresponding road, that is to say during a green light period as herein defined.

When a vehicle passes over a traffic detector in the road, the traffic light awaits the passage of a further vehicle for a predetermined period of time. If during this time interval no vehicle passes, the light changes and the green light of the transverse road is turned on. lf, on the contrary, a vehicle passes, the waiting time is prolonged for another period. This period is termed herein the green prolongation time.

lt is well known that an adaptive traffic light control system with fixed prolongation does not completely satisfy. The present invention permits of obtaining variable green prolongation times. This is achieved by means of a simple traffic light system mainly characterized in that it comprises two assemblies each formed by the series combination of a resistor and a capacitor which provide quite difi'erent time constants. These two assemblies are connected in parallel across the terminals of a direct voltage source, the junctions of the resistor and the capacitor of each of them being interconnected by a diode or a similar element. The capacitors have quite different capacitances and are discharged at the beginning of the green light. .The capacitor included in the circuit having the lower time constant is maintained in the discharged state for a fixed period of time at the termination of which the capacitor starts being charged. The latter capacitor is being discharged instantaneously during the passage of a vehicle over an appropriate detector located in the road, whereas the other capacitor is charged constantly and fairly slowly so that the green prolongation time (after the minimum time) which terminates when the voltage across the latter capacitor attains a given value, will vary in accordance with the volume of the traffic between two extreme values, one ofwhich is low.

The detailed description given hereinafter by way of nonlimiting example with reference to the accompanying drawing will show how the invention may be carried into effect.

F IG. 1 shows a circuit according to the invention. C, and C designate two capacitors. The capacitance of C is higher than that of C,. Each of these capacitors is connected by one electrode to a fixed negative direct voltage source -V. The two other electrodes are connected by a diode 1D, with the pass direction from C, to C The terminals of C, are connected to a switch I, of any suitable type, which is closed for a very short time when a road detector signals the passage of a vehicle approaching the light.

designates a variable potentiometer connected between the junction of C, and D, on the one hand and to a positive direct voltage source +V on the other hand.

P also denotes a variable potentiometer connected between the junction of C and D, on the one hand and to a positive direct voltage source +V on the other hand. The resistance of potentiometer P is greater than that of potentiometer P,.

When the light turns green, the capacitors C, and C, are discharged. During a predetermined minimum time the capacitor C, is maintained in the discharged state. The capacitor C is charged through the potentiometer P At the end of the minimum time the capacitor C, starts being charged through the potentiometer P,.

At each passage of a vehicle along a road detector of any suitable type, the interruptor I is closed and then opened almost instantaneously. The capacitor C, is thus discharged almost instantaneously.

When the voltage across the capacitor C, becomes equal to t the voltage across the capacitor C the capacitor C, is charged with an approximate time constant P,(C,+ which is higher than P, C,. Actually, the capacitors C, and C, are charged with a time constant:

However, since P P,, the foregoing approximation results.

When the voltage across C attains a given value, the green prolongation time is terminated.

Obviously, when passages of vehicles are very frequent, the green prolongation time maintains a fairly high value; if the vehicles are well spaced apart, the green prolongation time is less long. If no vehicle passes, the prolongation is not longer than a duration of the order of P,(C,+C

On the other hand, when the traffic is constant for a given time interval, the capacitor C is charged to a gradually increasing value and each prolongation after the discharge of the capacitor C, tends to shorten. When the voltage across capacitor C reaches a predetermined value, the green prolongation time is terminated.

These results are illustrated partly in the graph of FIG. 2, which shows the green prolongation time adjusted during the passage of a vehicle as a function of the. instant of said passage.

On the abscissa is plotted the time t, of said instant of passing and on the ordinate is plotted the time 1 of the duration of the green prolongation.

To zero on the abscissa corresponds the beginning of the green light.

The abscissa of the vertical straight line D, corresponds to the minimum green time and that of the vertical straight line D, to the maximum green time.

The straight line D, corresponds to a high traffic density (total prolongation time is fixed) and the straight line D, to practically zero traffic. The broken line D, corresponds to an intermediate case of average traffic.

lclaim:

1. ln a vehicular traffic light control system having a vehicle sensing device along the road, the improvement comprising a source of direct voltage having first and second terminals, a first resistorand a first capacitor serially connected between said first and second voltage terminals, a second resistor and a second capacitor serially connected between said first and second voltage terminals, the first series combination having a shorter RC time constant than the second series combination, a current rectifying element interconnecting the junctions of the resistor and capacitor of each series combination with its pass direction from said first series combination to said second series combination, said second capacitor being charged during the green light period of the traffic light at a constant rate so long as the voltage across the first capacitor is below the second capacitor voltage, said first capacitor being charged during said green light period, and means controlled by said sensing device for discharging the first capacitor substantially instantaneously upon the passage of a vehicle relative to the sensing device whereby the green light period is prolonged in accordance with the volume of traffic, the green light period being terminated when the voltage across the second capacitor attains a given value.

2. A control system as claimed in claim 1 wherein each of said resistors is connected to the first voltage terminal and each capacitor is connected to the second voltage terminal and wherein said rectifying element is a diode.

3. A control system as claimed in claim 1 further comprising means for discharging the first capacitor at or before the start of a green light period.

4. A control system as claimed in claim 3 including means for holding said first capacitor in the discharged state for a ri-G ai begins to charge at a faster rate than the second capacitor.

5. A control system as claimed in claim 4 wherein each of said resistors is connected to the first voltage terminal and each capacitor is connected to the second volta and wherein said rectifying element is a diode.

ge terminal 

1. In a vehicular traffic light control system having a vehicle sensing device along the road, the improvement comprising a source of direct voltage having first and second terminals, a first resistor and a first capacitor serially connected between said first and second voltage terminals, a second resistor and a second capacitor serially connected between said first and second voltage terminals, the first series combination having a shorter RC time constant than the second series combination, a current rectifying element interconnecting the junctions of the resistor and capacitor of each series combination with its pass direction from said first series combination to said second series combination, said second capacitor being charged during the green light period of the traffic light at a constant rate so long as the voltage across the first capacitor is below the second capacitor voltage, said first capacitor being charged during said green light period, and means controlled by said sensing device for discharging the first capacitor substantially instantaneously upon the passage of a vehicle relative to the sensing device whereby the green light period is prolonged in accordance with the volume of traffic, the green light period being terminated when the voltage across the second capacitor attains a given value.
 2. A control system as claimed in claim 1 wherein each of said resistors is connected to the first voltage terminal and each capacitor is connected to the second voltage terminal and wherein said rectifying element is a diode.
 3. A control system as claimed in claim 1 further comprising means for discharging the first capacitor at or before the start of a green light period.
 4. A control system as claimed in claim 3 including means for holding said first capacitor in the discharged state for a predetermined period of time whereafter said first capacitor begins to charge at a faster rate than the second capacitor.
 5. A control system as claimed in claim 4 wherein each of said resistors is connected to the first voltage terminal and each capacitor is connected to the second voltage terminal and wherein said rectifying element is a diode. 